MARJAH, Afghanistan - After a fierce gunfight, U.S. Marines seized a strongly defended compound Friday that appears to have been a Taliban headquarters - complete with photos of fighters posing with their weapons, dozens of Taliban-issued ID cards and graduation diplomas from a training camp in Pakistan.

Insurgents who had been using the field office just south of Marjah's town center abandoned it by the end of the day's fighting, as Marines converged on them from all sides, escalating operations to break resistance in this Taliban stronghold in southern Helmand province.

Marines from Lima Company, 3rd Battalion, 6th Marines fought their way south from the town center Friday after residents told them that several dozen insurgent fighters had regrouped in the area.

Throughout the day, small groups of Taliban marksmen tried to slow the advance with rifle fire as they slowly fell back in face of the Marine assault.

"They know that they are outnumbered ... and that in the end they don't have the firepower to compete with us conventionally," said Capt. Joshua Winfrey of Tulsa, Okla., commander of Lima Company, 3rd Battalion, 6th Marines.

As the Marines advanced, they found rows of abandoned bunkers dug alongside an irrigation canal that the Taliban had used to fire on them the day before. Located at a crossroads, the five abandoned bunkers, camouflaged under a layer of mud, looked out across an open field. In the near distance, large stones had been set up to help the Taliban site in on their targets.

Just behind the bunkers, the Marines found a compound, surrounded by a mudbrick wall, typical of family homes in the town.

Inside the compound, where a few chickens still wandered, Marines uncovered dozens of Taliban-issued ID cards, official Taliban letterhead stationery and government stamps.

They also found graduation diplomas from an insurgent training camp in Baluchistan, an area of southern Pakistan that borders Helmand province, along with photos of fighters posing with AK-47 assault rifles.

The insurgents had fled with their weapons and ammunition. The Marines said they'd been coming under fire all day - but never saw any of the elusive gunmen, who retreated to resume hit-and-run tactics using snipers and small gun squads to harass Marine lines.

Lima Company's advance was part of a move by several Marine companies to converge on a pocket of Taliban fighters from all four directions. The Marines believe they've cornered what appeared to be a significant Taliban fighting force.

"It seems that it's their last stand," Winfrey said.

NATO said one service member died Friday in a small-arms attack but did not identify the victim by nationality.

Six coalition troops were killed Thursday, NATO said, making it the deadliest day since the offensive began Feb. 13. The death toll for the operation stands at 12 NATO troops and one Afghan soldier. Britain's Defense Ministry said three British soldiers were among those killed Thursday.

No precise figures on Taliban deaths have been released, but senior Marine officers say intelligence reports suggest more than 120 have died. The officers spoke on condition of anonymity because they were not authorized to release the information.

The Marjah offensive is the biggest since the 2001 U.S.-led invasion of Afghanistan and a test of President Barack Obama's strategy for reversing the rise of the Taliban while protecting civilians.

Marjah, 360 miles southwest of Kabul, has an estimated population of 80,000 and had been under Taliban control for years.

Before dawn on Saturday, about two dozen elite Marines were dropped by helicopter into an area where skilled Taliban marksmen were known to operate, an officer said.